1. Field of the Invention
The present invention relates to telecommunications and, more particularly, to paging of wireless devices such as cellular telephones for instance.
2. Description of Related Art
a. Paging Slot Cycles
In a cellular communication system, an air interface paging channel may be divided into a number of timeslots in which a base station can transmit a page message to a mobile station. In order to conserve mobile station battery power, each mobile station will typically be arranged to operate at a given “slot cycle index” (SCI), which defines the frequency at which the mobile station will wake up and check the paging channel for a page. For instance, under cdma2000, a mobile station operating at slot cycle index 0 (zero) would wake up and check for a page every 1.28 seconds, whereas a mobile station operating at slot cycle index 2 would wake up and check for a page every 5.12 seconds.
When a mobile station first registers with a cellular serving system, the mobile station and the serving system will negotiate for use of a given slot cycle index. Thereafter, the serving system will then page the mobile station on a timeslot that the mobile station is set to check. For instance, if a mobile station is operating at slot cycle index 0, then the base station will page the mobile station in a timeslot that is some multiple of 1.28 seconds from time t=0, whereas if the mobile station is operating at slot cycle index 2, then the base station will page the mobile station in a timeslot that is some multiple of 5.12 seconds from time t=0.
Generally speaking, the more often a mobile station wakes up to check the paging channel for a page, the more quickly the mobile station's battery will drain. Therefore, it is generally desirable for a mobile station to operate at a relatively slow (or infrequent) slot cycle, such as at slot cycle index 2 for instance. Slot cycle index 2 is generally adequate to support paging for incoming telephone calls.
However, in some instances, it makes sense for a mobile station to operate at a faster slot cycle, such as slot cycle 0. By way of example, if a mobile station is operating in a push-to-talk (PTT) mode, in which another user might seek to establish “instant” communication with the mobile station, it would be best for the mobile station to operate at a faster slot cycle, so as to reduce latency in setting up such a communication.
In practice, most of the mobile stations operating in a wireless carrier's system will be set to a relatively slow slot cycle, such as slot cycle index 2, since most of the mobile stations will be engaging in normal telephone calls rather than PTT-type communication. However, some mobile stations in the carrier's system might be set to a relatively fast slot cycle, such as slot cycle index 0, in order to most usefully engage in PTT-type communication.
b. Re-Paging after Failure
When a mobile station receives a page, it will conventionally respond to the serving system with a page response message. However, in some cases, the serving system will not receive the page response, which would mean that a failure of some sort occurred in paging the mobile station. The failure could result from the mobile station being temporarily out of coverage or for some other reason. When that happens, the serving system may try re-paging the mobile station.
A serving system will wait to receive a page response for a given time interval after paging a mobile station, and if the serving system does not receive a page response from the mobile station by the expiration of that time interval, the serving system will then conclude that paging failed and will re-page the mobile station on the next timeslot that the mobile station is set to monitor. For purposes of this description, the time interval that the serving system waits before deciding that paging has failed may be referred to as a “failure-interval.”
In existing cellular communication systems, a serving system will apply a fixed failure-interval between a first and second page attempt for any mobile station operating in the system, regardless of the slot cycle index at which the mobile station is operating. Generally, the carrier would use a fixed failure interval that is believed to be long enough to allow a typical condition giving rise to a paging failure to go away. (For instance, the interval might be long enough that, if the mobile station had temporarily moved out of coverage, the mobile station is likely to have moved back into coverage by the time the interval expires.) Further, the interval would preferably be long enough to avoid unnecessarily increasing congestion on the paging channel (by paging too often).
By way of example, a carrier might use a fixed failure-interval of 6 seconds. Thus, if the mobile station operates at slot cycle index 0 and the serving system pages the mobile station at time t=0 and receives no response by time t=6, then the serving system would re-page the mobile station on the next possible slot, i.e., the slot that occurs at time t=6.4 (that is, 5×1.28). On the other hand, if the mobile station operates at slot cycle index 2 and the serving system pages the mobile at time t=0 and receives no response by time t=6, then the serving system would re-page the mobile station on the next possible slot, i.e., the slot that occurs at time t=10.24 (that is, 2×5.12).